The present invention relates to methods and systems for controlling a steering system, and more particularly to average friction learning and average friction change estimation in an electric power steering system.
In Electric Power-assisted Steering (EPS) systems, system friction is an important characteristic which can significantly affect the EPS performance and steering feel. During system calibration, the system friction is carefully addressed in order to achieve premium steering feel. However, the system friction is not constant over the EPS system lifespan, and there are many factors affecting the system friction. Some of these factors have a long term impact on the system friction, such as gear wear and grease degradation. Some other factors can affect the system friction level in a relatively rapid way, such as temperature.
A hysteresis loop is commonly used in describing the characteristics of a steering system. In an electric power-assisted steering system, many EPS tuning and compensation values are related to a hysteresis loop. Depending on the variables in the hysteresis loop, some EPS system characteristics can be derived directly from the hysteresis loop. For example, from a hysteresis loop formed by a total steering torque in a column coordinate versus a hand wheel angle, total EPS friction can be approximated as half of the hysteresis height at a given hand wheel angle.
EPS hysteresis loops are traditionally obtained offline during calibration, and all EPS calibration values related to the hysteresis loops are then derived and programmed in persistent memory, such as erasable programmable read only memory (EPROM) or flash memory. However, the EPS system characteristics will change due to environmental changes. At different life stages of an EPS system, a set of calibration parameters which was optimized for the original EPS system will not necessarily continue to give optimal performance.
Efforts have been made to compensate for the system friction, such as hysteresis compensation and temperature compensation functions. For hysteresis compensation, a vehicle speed-dependent curve is used to generate a certain amount of compensation torque to obtain desired steering feel, and this speed-dependent curve is calibrated during system calibration for a given vehicle model for the system friction level at that time. During the life of the EPS system, as the friction changes, the then-to-be-the-best speed dependent hysteresis compensation curve becomes sub-optimal, and an adjustment of the hysteresis compensation curve is desired.